2010 United States Senate Democratic primary election in Pennsylvania

The Democratic Party primary for the 2010 United States Senate election in Pennsylvania took place on May 18, 2010, when Congressman Joe Sestak defeated incumbent Arlen Specter, which led to the end of Specter's five-term Senatorial career. Just before the start of the primary campaign, after serving in the Senate as a Republican for 29 years, Specter had switched to the Democratic Party in anticipation of a difficult primary challenge by Pat Toomey; Toomey ultimately defeated Sestak in the general election. Political observers and journalists described the race between Specter and Sestak as one of the bitterest and most-watched of all the 2010 primary elections. Although Sestak was initially urged to run by the Democratic establishment, Specter gained broad support from Democrats after he switched parties. Prominent political figures like President Barack Obama and Pennsylvania Governor Ed Rendell later tried to sway Sestak from continuing the race, fearing he would damage Specter's chances in the general election. Former President Bill Clinton offered Sestak a position in the Obama administration if he withdrew his candidacy, an offer Republicans would later criticize. Sestak refused to drop out and criticized Specter's party switch as an opportunistic move aimed solely at self-preservation. Nevertheless, Sestak struggled to overcome problems stemming from low name recognition and Specter's support from such individuals as Joe Biden and Harry Reid, and organizations like the AFL–CIO and Pennsylvania Democratic Committee. Specter led Sestak by more than 20 percentage points for most of the race. However, this lead narrowed significantly in the final month of the campaign, when Sestak concentrated his funds and efforts on television commercials questioning Specter's Democratic credentials. Specter grew more critical of Sestak as the race progressed, attacking his House attendance record, accusing him of failing to pay his staffers minimum wage and alleging he was demoted while serving in the U . Navy for creating a poor command climate. Political observers said Sestak's commercials played a major part in his victory in the primary election. A national swing in momentum toward Republicans and against incumbents ultimately harmed Specter's chances.